The present invention refers to a system and to the relative control method for sensorless drives with an induction motors.
Currently field oriented, or vector controlled, induction motor drives are widely used in various industrial applications, such as machine tools, pumps, conveyor belts and others, wherein they are progressively replacing the traditional, direct current, electric drive means, thanks also to their improved ruggedness, reduced maintenance and to the lower production costs.
Field oriented control is based on the independent control of the two stator current vector components which are defined in a reference system synchronous with the rotor flux. The instantaneous position of the rotor flux, necessary for implementing field oriented control, can be measured using Hall effect flux sensors. However, such sensors do not provide, in practice, the required precision; further, their instalment often creates problems, whereby it is preferable to obtain the position of the rotor flux in an indirect way, through calculation of the equations of a mathematical model of the motor.
This last technique is today, by far the most used, even though it requires an angular position transducer of the machine shaft with a resolution equal at least to 8 bits and although the so obtained results are extremely sensitive to the rotor resistance changes caused by the temperature.
Sensitivity to the parameter variation and the need to introduce within the drive system a relatively expensive position sensor have initiated, the development of alternative techniques, of the xe2x80x9csensorlessxe2x80x9d type, suitable to determine the rotor flux position without using a position transducer.
Sensorless vector control techniques are based on various principles, and the best known are the direct field oriented control techniques, wherein the rotor flux position is obtained from the back electromotive force (EMF).
Techniques based on such a principle can be easily implemented at a low cost, but they fail at low and zero rotor speeds.
More sophisticated, sensorless type, techniques based on injecting suitable reference signals and on measuring the high frequency current harmonics allow to reduce significantly the minimum motor speed allowed by the control; however also such techniques do not completely solve the problem of the field oriented control at very low or zero speeds.
On this regard, it has to be noted that the motor control at very low and zero speeds is required very frequently and, in particular, it is useful in electric traction applications, in robotics, in many machine tools of the new generation and, in general, in all applications where it is required to balance a load at standstill or to bring the tool back to a defined position.
A purpose of the present invention is therefore to realise a control system for induction motor drives able to overcome the above mentioned disadvantages and, in particular, to define a control system to realise xe2x80x9csensorlessxe2x80x9d type drive means, suitable to properly work at very low speeds or even at zero speed.
Another purpose of the present invention is to define a control method for induction motor drives able to determine the position of the airgap flux and, consequently, the rotor flux position, independently from the rotor speed.
A further purpose of the present invention is to realise a control system for induction motor drives and to indicate an implementation scheme using conventional circuits and computing devices easy to install and to use, at low costs compared to standard techniques and in consideration of the attained advantages.
According to the present invention, these purposes are achieved by defining a control system for induction motor drives and the relative control method to which we must refer for the sake of brevity.
Advantageously, the proposed invention, emploies high frequency test signals and is based on the evaluation of the effects of said signals on the zero sequence components of the stator voltage or current.
Such a feature allows to overcome the imprecision found in other techniques using high frequency signals, due to the fact that the high frequency fluxes follow different paths than the low frequency fluxes and that the high frequency current components must be separated from the low frequency ones.